The present invention relates to a method of and apparatus for continuously casting a metal strip and, more particularly, to a continuous casting method and apparatus in which a metal strip is produced by making a molten metal contact with an outer peripheral surface of a rotating cooled casting roll to solidify the molten metal in the form of a solidified metal strip on the outer peripheral surface of the cooled casting roll, and then pulling the solidified metal strip upwardly while separating the same from the outer peripheral surface of the cooled casting roll.
Continuous casting methods of the kind stated above are disclosed, for example, in the specification of U.S. Pat. No. 3,587,708, specification of German Pat. No. 2,234,699 and also in "Handbook on Continuous Casting" by E. Herrmann, Aluminum-Verlag Dusseldorf, 1980, pp 85-93.
A method so-called "Melt Extraction Solidification Method" has been known as a technic for continuously casting a metal strip at a high speed in the manner e plained above, by pulling and separating metal strip solidified at its one side from the rotating cooled casting roll. This known method will be explained hereinunder with specific reference to FIG. 1. A cooled casting roll 3 is partially immersed in the molten metal contained in a refractory vessel 1 and is rotated in the direction of an arrow C, so that a solidified metal strip 4 is formed on the peripheral surface of the cooled casting roll. The solidified metal strip is pulled in the direction of an arrow D in synchronization with the rotation of the cooled casting roll. A roll 5 and a knife 6 are used in combination for the control of contact and separation of the solidified strip with and from the cooled casting roll 3. This known method offers various advantages. For instance, a high-speed casting is attainable because there is no friction between the cooled casting roll 3 and the solidified metal strip 4. In addition, this method is suited to the production of thin metal strip because the solidification is made by cooling from one side. Furthermore, it is to be noted that a smooth casting surface of cast strip is obtainable particularly at the side thereof contacted with the outer peripheral surface of the cooled casting roll, because the outer peripheral surface is gently and smoothly brought into contact with the molten metal.
This known method, however, suffers from the following disadvantages. Namely, for the first point, the yield or rate of production of the metal strip is limited because the circumferential length over which the cooled casting roll 3 contacts the molten metal, i.e. the length of an arc between the points A and B shown in FIG. 1 is small and thus an amount of molten metal solidified per unit time is small. This is attributable to the facts that the level of the molten metal surface 8 cannot be raised above the level of the rotating shaft 7 of cooled casting roll 3 and that the area of contact between the side walls of the cooled casting roll 3 and the molten metal 2 is increased as the level of the molten metal surface 8 is raised to unnecessarily increase the heat input to the cooled casting roll 3.
For the second point, it is to be pointed out that the thickness of the solidified strip 4 tends to become non-uniform. This is atributable to the fact that a slight fluctuation in the level of molten metal surface 8 exerts an influence upon the contact length AB thereby changing the time length of contact between the molten metal 2 and the cooled casting roll 3.
For the third point, it is difficult to obtain the required cross-sectional shape of the solidified strip. Namely, as shown in FIG. 2, substantially L-shaped projections or lugs 9 tend to be formed to project from both side edges of the solidified metal strip 4 in the breadthwise direction of the solidified strip 4, unless a suitable measure is taken on the cooled casting roll 3 for preventing the molten metal from solidifying on the side surfaces of the roll 3. These lugs 9 seriously deteriorate the cross-sectional shape of the solidified strip. As one of the measures for preventing the molten metal from solidifying on the side surfaces of the cooled casting roll, it has been proposed to provide heat insulating members on the side surfaces of the cooled casting roll 3. This solution, however, imposes another problem that the thickness of the solidified strip 4 is reduced undesirably along both side edges thereof due to insufficient cooling at both side edges of the cooled casting roll 3, although it ensures a substantially uniform breadth of the solidified strip 4. In order to overcome this problem, it has been proposed to dispose heat insulating guide members along both side surfaces of the cooled casting roll 3. In this case, however, it is necessary to provide a gap between each guide member and side surface of the cooled casting roll 3, in order to avoid friction therebetween. It is quite difficult and almost impossible practically to provide such a gap between the heat insulating guide members and the side surfaces of the cooled casting roll 3 in such a manner as to prevent the molten metal from solidifying on the side surfaces of the cooled casting roll 3 in spite of the presence of the gap.